Convectively coupled gravity waves and moisture modes with top and bottom heavy vertical heating profiles

Bottom heavy (convective profile) and top heavy (stratiform profile) vertical heating profiles are imposed on a vertically resolved model in a nonrotating atmosphere developed by Raymond and Fuchs, 2007 (RF07).

The rainfall rate produced by the model is a function of tropospheric precipitable water and convective inhibition as in RF07. The sensitivity of precipitation rate to convective inhibition (CIN) is represented by two terms, one that represents the surface flux variations and the other representing the variations in wave-induced buoyancy anomalies just above the planetary boundary layer.

Two types of unstable modes are predicted by this model, the moisture mode and the gravity mode which maps onto the Kelvin mode in the rotating atmosphere. A slowly moving “moisture mode” moves eastward under the wind-induced surface heat exchange (WISHE) mechanism. The primary control of precipitation for the moisture mode is the precipitable water and the instability is governed by negative gross moist stability (GMS). The more rapidly propagating “gravity mode” moves eastward with a phase speed of approximately 18-19 m/s, which is close to the observed phase speeds of convectively coupled equatorial Kelvin waves. The precipitation is controlled primarily by changes in convective inhibition, which is also a cause for the instability in the convectively coupled gravity waves.

Four different scenarios are explored for the moisture mode and the gravity mode: bottom heavy vertical heating profile with positive and negative gross moist stability and top heavy vertical heating profile with positive and negative gross moist stability. The convectively coupled gravity waves have weak or no instability in the regime of bottom heavy heating profile while as we move towards the top heavy profile their growth rate increases and the growth rate peak moves to the more realistic zonal wavenumbers. The computed vertical structure, in particular the temperature structure for the Kelvin waves, matches that of the observed structure. The gravity waves are not sensitive to gross moist stability. The moisture mode develops in a bottom heavy heating profile regime while it does not appear in the top heavy one. It is strongly sensitive to gross moist stability. Identification of the moisture mode with actual tropical disturbances is still in question, but it might be a candidate mechanism for easterly waves, monsoon depressions and the Madden-Julian oscillation (MJO).